If you're one of the tens of millions of Yahoo users asked to upgrade your instant-messaging software this week, be on your toes: The update can open the door to unwanted PC houseguests--and setting changes--by default.

The newest free version of Yahoo Instant Messenger (YIM) boasts advanced Internet phone calling in a upgrade that comes "highly recommended" by Yahoo. By clicking "yes" to the update, a user can expect to get a slicker YIM interface with buttons to quickly chat, blog, swap photos or call someone online. It even has new smiley icons.

Those changes are what many might expect. What they may not expect are all the other tools they get when not paying attention.

By accepting Yahoo's "typical" installation of YIM with Voice, it will also download Yahoo's Search Toolbar with anti-spyware and anti-pop-up software, desktop and system tray shortcuts, as well as Yahoo Extras, which will insert Yahoo links into the Internet Explorer browser. The IM client also contains "live words," which will automatically show an icon when the user highlights words online and then hyperlink to Yahoo search results, definitions or translation tools. Finally, the installation will alter the users' home page and auto-search functions to point to Yahoo by default.

To avoid these changes, users must actively choose the "custom" installation and uncheck five boxes.

Yahoo spokeswoman Terrell Karlsten said that for avid Yahoo users, the included services are valuable and highlight the integration among all its tools.

"By setting it that way we're giving people choices. For people who want to download software in one fell swoop, they have that option. If they don't want it we give them the ability to customize it," Karlsten said.

Battle for the desktop
Privacy advocates called Yahoo's install tactics disappointing, yet part of a long history of guerrilla marketing among Internet companies promoting free software. Ray Everett-Church, a principal at privacy consultancy PrivacyClue, said in this case that Yahoo was going further than it ever had before.

"This is the first instance where you actually must go and do a custom install to control them from installing other software," Everett-Church said. "Most folks go to the default install and are not expecting to get a whole suite of unasked-for software. That's where the sneaky factor comes in."

Yahoo isn't the only company employing aggressive means to promote their applications and services, nor is it the first. Big companies including MSN and America Online and smaller firms like RealNetworks and Claria have long taken the opportunity to push other tools or features when users install their software.

The tactics are so common that people often forget about the technological battle being fought for control of their PC, in which Internet companies play offense and defense when it comes to changing preferences for home pages, e-mail, multimedia players and default Web searches.

"The hidden check boxes during installation are a common practice for companies trying to further extend their reach onto your computer desktop," Everett-Church said. "Companies have a responsibility to very clearly indicate what is being installed during that process and give a clear opportunity to reject software."

In this case, the YIM software triggers a miniature battle with other software makers. If people accept Yahoo's default home page and search settings and then want to set them back, Microsoft's IE contains an option under its "Tools" menu to "Reset Web settings." That selection, however, will not necessarily restore personalized settings. Rather, it will point people to Microsoft IE default settings, including the MSN home page and auto-search features.

Yahoo is persistent, however. If people set back their auto-search settings to MSN or Google, for example, Yahoo will send a pop-up message asking users if they would like to change it yet again to Yahoo Search.

The Sunnyvale, Calif.-based company has been testing the new version of YIM with advanced Net phone service since May and released it in early August. This week, it began pushing out a "recommended upgrade" notice to U.S. users, Karlsten said. It has included default home page and search changes since May 2004, when it released YIM 6.0.

Yahoo Messenger with Voice also includes shortcuts to IM from the desktop and system tray. The Yahoo Extras software will specifically add links into IE that point people to Yahoo services like e-mail and shopping. The "custom" installation of the software will let people opt not to receive the toolbar, shortcuts and extras.

Yahoo could be counting on the fact that most people won't care or notice the changes.

"There are a ridiculously high number of people who never budge off the default," Everett-Church said.